Additional Metatdata and project sites

I have been asked to complete the following and im a little unsure if it can be done and how.

A number of team sites would like to display a list of project sites which they are currently working on. The project sites would need to be assigned to departments as multiple deparatments can work on a single project.

I am aware you cannot add additonal metadata to a site but is there any other way other than creating a list and referecing it?

Thanks,